It seems obvious that we exist in the present. The past is gone and the future has not yet happened, so where else could we be? But perhaps we should not be so certain. Sensory information reaches us at different speeds, yet appears unified as one moment. Nerve signals need time to be transmitted and time to be processed by the brain. And there are events – such as a light flashing, or someone snapping their fingers – that take less time to occur than our system needs to process them.
By the time we become aware of the flash or the finger-snap, it is already history.Our experience of the world resembles a television broadcast with a time lag; conscious perception is not “live”. This on its own might not be too much cause for concern, but in the same way the TV time lag makes last-minute censorship possible, our brain, rather than showing us what happened a moment ago, sometimes constructs a present that has never actually happened.
Evidence for this can be found in the “flash-lag” illusion. In one version, a screen displays a rotating disc with an arrow on it, pointing outwards see “Now you see it…”. Next to the disc is a spot of light that is programmed to flash at the exact moment the spinning arrow passes it. Yet this is not what we perceive. Instead, the flash lags behind, apparently occuring after the arrow has passed.
One explanation is that our brain extrapolates into the future. Visual stimuli take time to process, so the brain compensates by predicting where the arrow will be. The static flash – which it cant anticipate – seems to lag behind.Neat as this explanation is, it cannot be right, as was shown by a variant of the illusion designed by David Eagleman of the Baylor College of Medicine in Houston, Texas, and Terrence Sejnowski of the Salk Institute for Biological Studies in La Jolla, California.
If the brain were predicting the spinning arrows trajectory, people would see the lag even if the arrow stopped at the exact moment it was pointing at the spot. But in this case the lag does not occur. Whats more, if the arrow starts stationary and moves in either direction immediately after the flash, the movement is perceived before the flash. How can the brain predict the direction of movement if it doesnt start until after the flash?
The explanation is that rather than extrapolating into the future, our brain is interpolating events in the past, assembling a story of what happened retrospectively Science, vol 287, p 2036. The perception of what is happening at the moment of the flash is determined by what happens to the disc after it. This seems paradoxical, but other tests have confirmed that what is perceived to have occurred at a certain time can be influenced by what happens later.
All of this is slightly worrying if we hold on to the common-sense view that our selves are placed in the present. If the moment in time we are supposed to be inhabiting turns out to be a mere construction, the same is likely to be true of the self existing in that present.This article appeared in print under the headline “When are you?”
via The self: You think you live in the present? – 20 February 2013 – New Scientist.